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Nitrate and ammonium have different effects on many biochemical and physiological processes in plants, and at high concentrations this can lead to markedly different growth responses. Most plant species show reduced growth, smaller leaves and a stunted root system when exposed to high ammonium concentrations, and in severe cases this leads to chlorosis. Although well known, ammonium toxicity is poorly understood and is generally considered to be the result of one or more of the following effects; (i) ammonium‐induced mineral nutrient deficiency, arising from the impaired uptake of metal ions; (ii) secondary growth inhibition arising from the acidification of the rooting medium; (iii) alterations in intracellular pH and osmotic balance; (iv) uncoupling of photophosphorylation from electron transport, following the accumulation of ammonium in leaves; and (v) altered polyamine and phytohormone metabolism. These hypotheses are reviewed in the light of the available literature and experimental evidence from own experiments. It is concluded that no mechanism on its own provides an adequate explanation of the available data.

Twenty-eight polyphenols (11 flavonoid derivatives and 17 hydroxycinnamic acid derivatives) were detected in different cultivars of the Chinese cabbage pak choi ( Brassica campestris L. ssp. chinensis var. communis) by HPLC-DAD-ESI-MS(n). Kaempferol was found to be the major flavonoid in pak choi, glycosylated and acylated with different compounds. Smaller amounts of isorhamnetin were also detected. A structural determination was carried out by (1)H and (13)C NMR spectroscopy for the main compound, kaempferol-3-O-hydroxyferuloylsophoroside-7-O-glucoside, for the first time. Hydroxycinnamic acid derivatives were identified as different esters of quinic acid, glycosides, and malic acid. The latter ones are described for the first time in cabbages. The content of polyphenols was determined in 11 cultivars of pak choi, with higher concentrations present in the leaf blade than in the leaf stem. Hydroxycinnamic acid esters, particularly malic acid derivatives, are present in both the leaf blade and leaf stem, whereas flavonoid levels were determined only in the leaf blade.

The effects of different treatments of salicylic acid (SA) on lipid peroxidation, chlorophyll fluorescence and antioxidant enzyme activity in seedlings of Cucumis sativa L. were studied before heat stress treatment, 36 h after heat stress and 24 h after recovery. Compared with the controls (foliar spray of distilled water), a foliar spray of 1 mM SA (SSA treatment) decreased electrolyte leakage and the concentration of H 2 O 2 and thiobarbituric acid reactive substances (TBARS). SSA treatment also enhanced maximum yield of photosystem II photochemical reactions (Fv/Fm) and the quantum yield of the photosystem II electron transport (UPSII) after both heat stress and recovery; however, adding 1 mM SA to the nutrient solution (ASA treatment) or both adding 1 mM SA to the nutrient solution and foliar spray of 1 mM SA as well (SSA + ASA treatment) had the opposite effects. SOD activity was stimulated by all SA treatments. CAT activity was stimulated by SSA treatment and inhibited by ASA and SSA + ASA treatments after heat stress and recovery. This suggest that SSA treatment can efficiently remove H 2 O 2 and decrease heat stress, and CAT plays a key role in removing H 2 O 2 in cucumber seedlings under heat stress, while more H 2 O 2 accumulates in ASA and SSA + ASA treatments and therefore induces serious oxidative stress. GPX, APX and GR showed higher activities in all SA treatments under heat stress, however, it appears that they were not key enzymes in removing H 2 O 2 in cucumber subject to heat stress.

Temporally regulated microRNAs have been identified as master regulators of developmental timing in both animals and plants. In plants, vegetative development is regulated by a temporal decrease in miR156 level, but how this decreased expression is initiated and then maintained during shoot development remains elusive. Here, we show that miR159 is required for the correct timing of vegetative development in Loss of miR159 increases miR156 level throughout shoot development and delays vegetative development, whereas overexpression of miR159 slightly accelerated vegetative development. The repression of miR156 by miR159 is predominantly mediated by MYB33, an R2R3 MYB domain transcription factor targeted by miR159. Loss of led to subtle precocious vegetative phase change phenotypes in spite of the significant downregulation of miR156. MYB33 simultaneously promotes the transcription of and, as well as their target, , by directly binding to the promoters of these three genes. Rather than acting as major players in vegetative phase change in Arabidopsis, our results suggest that miR159 and MYB33 function as modifiers of vegetative phase change; i.e., miR159 facilitates vegetative phase change by repressing MYB33 expression, thus preventing MYB33 from hyperactivating miR156 expression throughout shoot development to ensure correct timing of the juvenile-to-adult transition in Arabidopsis.

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